The most common objective of a geotechnical site investigation is to establish the ground conditions for development foundation design. Quite often however subsidence of existing buildings will require a small scale site investigation. At Environmental Investigations a geotechnical site investigation is often procured with an environmental site assessment in order to minimise the cost involving with the subsurface sampling including drilling, travel, etc. A geotechnical site investigation is intrusive and may include in-situ and laboratory tests.

Each site investigation is specifically designed to suit the clients requirements, the anticipated geology and hydrogeology and need for other remedial design advice or proposals for new development foundations. Many of the more common fieldwork and testing techniques are outlined in the following sections.

Fieldwork

Trial Pits

Extremely valuable if the depth of investigation is less than 3m. Trial pits allow a detailed examination of the ground conditions in-situ with some indication of stability and groundwater conditions, although disturbed samples are easier to obtain than undisturbed samples. Trial pitting is a relatively fast and efficient means of exploring sub-surface conditions particularly in areas of soft landscaping.

Auger Holes

Auger holes are normally made by hand-turning a very light auger into the ground or by using light power-auger equipment. However, whilst auger holes are effective in preliminary investigations they do not provide the depth or range of sampling and in-situ testing provided by conventional site investigation boreholes.

Window Samplers

A window sampler is a steel tube, usually about 1m long with a series of windows along the tube through which to view disturbed soil conditions or extract samples. A lightweight percussion hammer drives the sampler into the ground, which is then extracted with jacks. A depth of approximately 9m can be achieved using a sequence of progressively smaller diameter samplers.

Boreholes

Light percussion drilling (shell and auger) is the most commonly used method in Australia. Samples are often unsuitable for soil description on the basis that the drilling process will have changed the strength of the soil by remoulding it and increased the moisture content of the soil through lubrication. For this reason tube samples are often taken from boreholes at regular depths by driving a small diameter tube into the soil at the base of the borehole, thus extracting an undisturbed sample for laboratory analysis.

In Situ Testing

In-situ testing is typically undertaken on cohesionless soils where the strength characteristics may only be established on undisturbed in-situ samples.

Cone Penetration Test (CPT)

This technique involves hydraulically pushing a 10 or 15 cm2 cone into the ground at a standard rate of penetration (2cm/sec) and measuring its penetration resistance using modern electric cones. The equipment necessary for this type of investigation is housed in a large truck, so access may be restricted on some sites. However, the results from a CPT can be very valuable by providing a soil profile, estimates of the soil types encountered including consistency and density, and evidence of the presence of voids beneath the site.

Dynamic Probing

Much less sophisticated than CPT, dynamic probing involves driving a steel rod into the ground by using repeated blows of a hammer of a specified mass falling through a fixed distance. The number of blows required for each 100mm is recorded and plotted as a depth versus blow-count log. However, the information given by dynamic probing is very restricted and is difficult to interpret. Further investigations are usually necessary to supplement this test.

The Standard Penetration Test

Designed to provide the penetration resistance in the sub-surface, this test involves driving an open-drive sampler into the bottom of a borehole with repeated blows of a hammer falling a predetermined distance. The number of blows necessary to drive the sampler six increments of 75 mm are counted and recorded, giving the penetration resistance value. The test is undertaken on cohesionless soils such as gravels.

Other methods of testing and sampling are used depending on the soil type and conditions on site. A geotechnical site investigation is dependant upon site specific factors and as such will be designed specifically for the investigation in hand.

Laboratory Testing

Laboratory Testing is typically undertaken on cohesive soils i.e. clays, where an undisturbed sample may be retrieved from the field. Such testing is not appropriate to cohesionless soils such as gravels.

• Soil Classification

Particle Size

Distribution Plasticity

Index Moisture Content

• Consolidation

To assess the settlement characteristics

• Triaxial

To assess the compressive strength characteristics

Reporting

The results of the fieldwork and laboratory tests, if undertaken, are assessed.

For development purposes preliminary designs for foundations may be prepared. Sufficient information are provided to enable a structural engineer to design building foundations.

For diagnostic purposes the investigation would assist with identifying the cause of defects with a building superstructure allowing remedial solutions to be specified with confidence.